One of the most common ways to classify imaging systems is as to whether they are “passive” or “active.” Classical Galilean and Keplerian telescopes, and binoculars, are passive systems, which gather ambient light to magnify the image of objects of interest. Such systems are for surveillance and overall situational awareness purposes; as they will not highlight anything in particular within their field of view (other than a flash or bright reflection), and as they are not usable at night to distinguish dark images blended into a dark background. More modern night vision systems are also generally passive, utilizing ambient light reflected or thermal signals emitted from the object of interest being observed; such passive systems do not send out any energy; but, only act as a receiver. By way of contrast, imaging systems that emit energy, where the energy impinges upon and is reflected from the object for detection, is an active system.
Image intensifiers are well known for their ability to enhance night-time vision by multiplying the amount of ambient incident light, to produce a brighter, more intense image, yet, still, are passive systems. Such devices are particularly useful for enhancing images from dark regions for both industrial and military applications. The U.S. military uses image intensifiers during night time operations for viewing (i.e. detecting, recognizing, identifying) and aiming at targets that would not be otherwise visible. As stated, night ambient radiation (star and/or street lighting) is reflected from the target and the reflected energy is amplified by the image intensifier to make the target image visible. Other examples of image intensifier applications include: enhancing the night vision of pilots; providing night vision to suffers of retinitis pigmentosa (night blindness); and use in astronomical observation and photography.
A typical image intensifier, as disclosed in U.S. Pat. No. 5,146,077 to Caserta et al., includes an objective lens, which focuses visible and infrared radiation from an object onto a photocathode (the “signal”). The photocathode, a photoemissive wafer, is extremely sensitive to low-radiation levels of light in the 580-900 nm spectral range, emitting electrons in response to the electromagnetic radiation signal/energy focused thereon. Electrons emitted from the photocathode are accelerated toward a phosphor screen (an anode), which is maintained at a higher positive potential than the photocathode. A micro-channel plate (“MCP”), formed of many thousands of individual hollow glass fibers with a NiChrome electrode on either side, is located between the photocathode and the phosphor screen. A large 1000V potential is applied across this thin MCP, such that when electrons strike and pass through it, additional secondary electrons are released, amplifying the signal up to 30,000 times. Using multiple MCP layers, amplification of well over 1,000,000 times is possible. The phosphor screen converts the electron emission into visible light for observation by an operator.
The latest image intensifiers are referred to as third generation image intensifiers, use GaAs/CsO/AlGaAs photocathodes. They are more sensitive than prior photocathodes in the 800-900 nm spectral range—thereby providing higher low light sensitivity, greater than 900 μA/lm. As further disclosed in U.S. Pat. No. 5,146,077, to protect the GaAs photocathode from bombardment by positive ions emitted by the MCP, the MCP is coated with a thin aluminum oxide film. Third generation image intensifier tubes are manufactured by a variety of sources, including but not limited to Litton Corporation and ITT Corporation, and are incorporated into products such as the AN/PVS-14 Monocular Night Vision Device (MNVD). The AN/PVS-14 is used around the world by the U.S. and NATO armed forces.
A very general, active imaging system, incorporated herein by reference, is disclosed in U.S. Pat. No. 6,603,134, to Wild et al., wherein a radiant energy source, including light energy, radio frequency energy, microwave energy, acoustical energy, X-ray energy, or heat energy, is retroreflected from an object to detect that object. Retro-reflection is defined by Wild et al. as a reflector wherein incident rays or radiant energy and reflected rays are parallel for any angle of incidence within the field-of-view. It is also disclosed that a characteristic of a retroreflector is that the energy impinging thereon is reflected in a very narrow beam, a characteristic observed when such energy impinges the human eye or optical instruments, such as binoculars, telescopes, periscopes, range finders, cameras, and the like. Such a reflected, parallel, very narrow beam, can be termed to be collimated—wherein the rays within the beam are nearly parallel and spread slow with minimal dispersion.
U.S. Pat. Publication 2005/0033186, by Nordstrom et al., discloses an active system wherein the illuminating beam and the receiving beam reflected from the object of interest lie essentially along the same line of sight. By scientific definition, this system would be a “monostatic” system. In contrast, a bistatic system is one in which the illuminating beam is focused on the object from a source location and the light that is reflected, backscattered, or emitted from the object is received by an optical system situated a certain distance from the illumination source. In this configuration, the angle between the source and emitter relative to the object being illuminated is referred to as the “bistatic angle.” By logical extension, therefore, when the bistatic angle is zero, the system is defined as “monostatic.” As disclosed in this published patent application, for many applications, the bistatic configuration is not useful. For example, with a bistatic observation, contours within the object may cause shadowing of the response from the surface of the object to the receiver, or may cause overlap of the receiver line of sight, and the illumination line of sight to fall off the surface. It is further understood that misalignment problems can be overcome by the use of monostatic optical configurations and as discussed herein, any significantly increased angle from a monostatic configuration can significantly reduce the retroreflection detection of sniper scopes or other optics.
There is a need in the art to observe potentially critical objects, such as sniper scopes, which are often not distinguishable from the background environment by current passive or active retroreflection based optics, even those including 3rd generation image intensifiers or the like. Further, there is especially such a need for detection of sniper scopes during the daylight, where current 3rd generation image intensifiers, and the like, are not intended to function
The above objectives as well as other objectives, features, and advantages of the present invention will become more apparent from the following detailed discussion when considered in conjunction with the drawings and claims presented.